Yougang Tang

Heave-roll-pitch coupled nonlinear internal resonance response of a spar platform considering wave and vortex exciting loads

Many studies have been done on the heave-pitch unstable coupling response for a spar platform by a 2-DOF model. In fact, in addition to the heave and pitch which are in one plane, the nonlinear unstable motion will also occur in roll. From the results of the experiments, the unstable roll motion plays a dominant role in the motion of a spar platform which is much stronger than that of pitch. The objective of this paper is to study 3-DOF coupling response performance of spar platform under wave and vortex-induced force. The nonlinear coupled equations in heave, roll and pitch are established by considering time-varying wet surface and coupling. The first order steady-state response is solved by multi-scales method when the incident wave frequency approaches the heave natural frequency. Numerical integration of the motion equations has been performed to verify the first-order perturbation solution. The results are confirmed by model test. There is a saturation phenomenon associated with heave mode in 3-DOF systems and all extra energy is transferred to roll and pitch. It is observed that sub-harmonic response occurs in roll and pitch when the wave force exceeds a certain value. The energy distribution in roll and pitch is determined by the initial value and damping characteristics of roll and pitch. The energy transfers from heave to pitch and then transfers from pitch to roll. Due to the influence of the low-frequency vortex-excited force, the response of roll is more complicated than that of pitch.

Model Test for Lateral Soil Resistance of Partially Embedded Subsea Pipelines on Sand during Large-Amplitude Lateral Movement

ABSTRACT Wang, Z.; Tang, Y.; Feng, H.; Zhao, Z., and Liu, H., 2017. Model test for lateral soil resistance of partially embedded subsea pipelines on sand during large-amplitude lateral movement. The lateral soil resistance exerted on partially embedded pipes during large-amplitude lateral movement is the key parameter for the analysis of controlled lateral buckling attributable to axial compressive thermal stress. A series of large-scale model tests were performed to investigate the responses of pipe segment with different initial pipe embedment and different pipe weight during large-amplitude lateral movement. The lateral and vertical displacement and lateral soil resistance were measured during the test. The test results showed that breakout resistance and the trajectories of model pipe depend on initial pipe embedment and pipe weight; however, residual resistance is influenced only by pipe weight. In addition, the lateral soil resistance is idealized to two types of the lateral soil-resistance models on the basis of different initial embedment depth, namely the trilinear and quadlinear soil-resistance models, for deeper and shallower initial pipe embedment, respectively. A modified formula is presented to calculate breakout resistance. Finally, according to the failure mechanism of pipe soil interaction during lateral movement, a new formula for predicting residual resistance has been developed. The predicted results are well fitted with test results.

Effects of second-order wave forces and aerodynamic forces on dynamic responses of a TLP-type floating offshore wind turbine considering the set-down motion

A coupled 6 degree of freedom (DOF) aero-hydro-restoring model is developed to study the dynamics of a tension leg platform (TLP)-type offshore wind turbine. This model includes second-order wave loads, pitch control strategies, and effects of platform motion on aerodynamic performance. The second-order wave force is calculated using the full-field quadratic transfer function. The coupled effects of the horizontal motions (such as surge, sway, and yaw motions) and the set-down motion are taken into consideration through the nonlinear restoring matrix. Different load scenarios are chosen to simulate the platforms dynamic response and the rotors aerodynamic performance in the time domain. The analysis shows that second-order wave forces will induce slow-drift and springing resonances. Due to the large motion in surge, a significant set-down motion which is an important part of heave motion is induced by the second-order difference-frequency force. When different wind types are chosen, the aerodynamic damp...

Effects of Second-Order Sum- and Difference-Frequency Wave Forces on the Motion Response of a Tension-Leg Platform Considering the Set-down Motion

This paper presents a study on the motion response of a tension-leg platform (TLP) under first- and second-order wave forces, including the mean-drift force, difference and sum-frequency forces. The second-order wave force is calculated using the full-field quadratic transfer function (QTF). The coupled effect of the horizontal motions, such as surge, sway and yaw motions, and the set-down motion are taken into consideration by the nonlinear restoring matrix. The time-domain analysis with 50-yr random sea state is performed. A comparison of the results of different case studies is made to assess the influence of second-order wave force on the motions of the platform. The analysis shows that the second-order wave force has a major impact on motions of the TLP. The second-order difference-frequency wave force has an obvious influence on the low-frequency motions of surge and sway, and also will induce a large set-down motion which is an important part of heave motion. Besides, the second-order sum-frequency force will induce a set of high-frequency motions of roll and pitch. However, little influence of second-order wave force is found on the yaw motion.

Study of the natural vibration characteristics of water motion in the moon pool by the semi-analytical method

The three-dimensional natural vibration characteristics of water inside a moon pool of an ocean structures are studied. The governing equations are derived based on the linear potential flow theory, and the boundary condition of the total opening bottom suggested by Molin is adopted. A semi-analytical method is used to solve the governing equations, and the natural frequencies and the motion modes are obtained. Two types of motions are studied: (1) the piston motion in the vertical direction, and (2) the sloshing motion of the free surface. The influences of moon pool’s structural parameters on the natural frequencies, and the modal shapes are analyzed.

The Dynamic Response of Floating Foundation for 1.5MW Wind Turbine in 30m Water Depth

The floating foundation is designed to support a 1.5MW wind turbine in 30m water depth. Considering the viscous damping of foundation and heave plates, the amplitude-frequency responses characteristics of the foundation are studied. Taking the elastic effect of blades and tower, the classic quasi-steady blade-element/ momentum (BEM) theory is used to calculate the aerodynamic elastic loads. Coupled dynamic model of turbine-foundation-mooring lines is built to calculate the motion response of floating foundation under Kaimal wind spectrum and regular wave by using the FAST codes. The model experiment is carried out to test damping characteristics and natural motion behaviors of wind turbine system, as well as the motion responses considering joint of wind and wave and only with wave. It is shown that the wind turbine system can avoid the harmonic vibration by the wind and wave, in addition, the floating foundation heave is induced by wave, while surge motion is induced by wind, and the action of wind and wave is of significance for pitch.Copyright

Stochastic Chaotic Roll Motion of Ship With Water on Deck

The stochastic chaotic motion of ship rolling is studied considering the static effect of water on deck. The wave excitation is treated as periodic excitation perturbed by white noise and the parameter region for chaotic motion is determined by Melnikov mean-square criterion. The Poincare’ maps are constructed and the probability density functions are calculated using path integral method. It is found that the chaotic motion can be restrained by increasing ship damping and ship rolling response converges to two attraction regions. In the non-chaotic parameter region, ship rolls within one attraction region. In the chaotic parameter region the probability density function has two connected peaks and ship rolling jumps randomly from one attraction region to another.Copyright